There is presently available a variety of equipment used to form a sheet of plastic material into a predetermined shape or design for use in signs, ceiling panels and other similar items. These machines usually include a heating oven, a table which includes a mold about the sheet of plastic is to be formed, and a frame which supports the sheet of plastic so that it can first be exposed to the heating oven until it is plasticized and then pulled down over the mold and formed. Typical example of equipment of this general sort are disclosed in Shuman U.S. Pat. No. 3,553,787, issued Jan. 21, 1971, and Shuman U.S. Pat. No. 4,018,551, issued Apr. 19, 1977.
The aforesaid frames generally have an open, rectangular shape, so that the peripheral edges of the plastic sheet can be supported thereat with the major portion of the plastic sheet extending across the rectangular opening defined by the frame, whereby the frame can be moved to a position in or adjacent to the heating elements of the oven until the sheet is plasticized and then pulled down over the sides of the mold with the mold protruding up through the frame opening to form the portion of the plastic sheet extending thereacross. Because the plastic sheet is somewhat flaccid and must be securely held by the frame during the forming operation, clamping surfaces are usually provided to extend along the plastic sheet supporting surfaces of the frame and are selectively pressed against such supporting surfaces to clamp and hold the edges of the sheet of plastic therebetween.
While these clamps serve to hold the plastic sheet against the frame during the forming operation, they also have the disadvantage of covering a portion of the plastic sheet while it is being heated so that the held edges of the sheet do not become fully heated and, as a result, become distorted. Additionally, the clamps are directly exposed to the heating elements so that the clamps actually absorb some of the heat generated, thereby reducing the quantity of heat applied to the exposed portion of the plastic sheet. More specifically, when the frame having a plastic sheet clamped thereto is raised to a position adjacent the heating elements of the oven, the central portion of plastic sheet becomes plasticized after a predetermined length of time. However, the edges of the sheet which are being pressed and held between the frame and the clamps are not fully exposed to the heat generated by the heating elements, and, as a result, distortion of the plastic at these edges occurs, usually in the form of undulations extending along these edge portions.
Aside from being unsightly, the distorted edge portions of the plastic sheet will, in most instances, present a significant problem to the user because the distorted edge portions will not fit properly in a mounting frame or support. For example, formed plastic sheets used as signs are usually held in a mounting frame with the edges of the sheet being inserted in relatively narrow slots or grooves that will receive only a flat surface configuration. Accordingly, it is now common practice to simply cut off all, or substantially all, of the distorted edges of the plastic sheet, and rely on the mold to form the necessary flat edge surface for mounting the formed plastic sheet into a mounting frame. This practice is time-consuming and costly and it results in a substantial waste of plastic since up to ten percent of the plastic may be cut off and discarded.
In an effort to alleviate the aforesaid warping and distortion at the clamped side edges of the plastic sheet, it has heretofore been proposed, as disclosed for example in the above-mentioned U.S. Pat. No. 4,018,551, to provide openings in the clamping surface for allowing heat from the oven to reach the clamped edges of the plastic sheet and cause a more equal heating of the plastic sheet. While this proposal served to correct the distortion problem to some extent, it has been found, nevertheless, that some distortion of the side edges can still occur in plastic forming operations.
In accordance with the present invention, a frame is provided in which heat is applied directly to the supporting surface to which the side edges of the plastic sheet are clamped to thereby substantially equalize the heating of the sheet and substantially eliminate any warping or distortion at the side edges thereof.